Plans to release water could keep up to 20,000 homes flooded for up to 15 days.

AP: Gregory Bull

The US Coast Guard says it has rescued at least 3,000 people from Tropical Storm Harvey's floodwaters in the past 48 hours, with the death toll now at more than 40 people and rising.

A week after it slammed into Texas, Harvey has retained enough rain-making power to raise the risk of flooding as far north as Indiana, while in Houston, officials tried to safeguard parts of their devastated city by intentionally flooding others.

Mayor Sylvester Turner announced plans to release water from two reservoirs that could keep as many as 20,000 homes flooded for up to 15 days.

Some of the affected houses have several metres of water in them, and the water reaches to the rooftops of others, district meteorologist Jeff Lindner said.

Mr Turner pleaded for more high-water vehicles and search-and-rescue equipment as the nation's fourth-largest city continued looking for any survivors or corpses that might have escaped notice in flood-ravaged neighbourhoods.

Search teams quickly worked their way down streets, sometimes not even knocking on doors if there were obvious signs all was well, such as organised debris piles, full cans of trash on the curb, or neighbours confirming residents had evacuated.

Authorities considered it an initial search, though they did not say what subsequent searches would entail or when they would commence.

Mr Turner also asked the Federal Emergency Management Agency (FEMA) to provide more workers to process applications from thousands of people seeking government help.

Harvey victims expect FEMA to work "with the greatest degree of urgency", he told CBS.

Mr Turner said he would request a preliminary aid package of $US75 million for debris removal alone.

More than 1,500 people were staying at shelters in Louisiana, and that number included people from communities in Texas.

The state opened a seventh shelter Friday in Shreveport for up to 2,400 people, Shauna Sanford, said a spokeswoman for Louisiana Governor John Bel Edwards.

The Texas city of Beaumont, home to almost 120,000 people near the Louisiana state line, was trying to bring in enough bottled water for people who stayed behind after a water pumping station was overwhelmed by the swollen Neches River.

One Houston man returned to his flooded house to discover a nearly three-metre alligator inside, KTRK-TV reported on Friday.

It took four men to carry away the reptile, whose mouth was taped shut.

Thousands of properties destroyed

Authorities raised the death toll from the storm late on Thursday, while rescue workers conducted a block-by-block search of tens of thousands of Houston homes.

An estimated 156,000 dwellings in Harris County, or more than 10 per cent of all structures in the county database, were damaged by flooding, according to the flood control district for the county, which includes Houston.

Mr Lindner called that a conservative estimate.

Figures from the Texas Department of Public Safety indicated that nearly 87,000 homes had major or minor damage and at least 6,800 were destroyed.

Governor Greg Abbott warned it could take years for Texas to "dig out from this catastrophe".

US President Donald Trump tweeted that there was still "so much to do" in Texas' recovery.

With three months remaining in the official Atlantic hurricane season, a new storm, Irma, had strengthened on Friday into a category three storm.

It remains hundreds of miles from land but was forecast to possibly hit Puerto Rico, the Dominican Republic and neighbouring Haiti by the middle of next week.